The present disclosure relates to control devices that control a vehicle drive device.
Hybrid vehicles using both an internal combustion engine and a rotating electrical machine as driving force sources for wheels have been put to practical use. A device disclosed in Japanese Patent Application Publication No. 2008-114848 is known as an example of vehicle drive devices that are used for such hybrid vehicles. The vehicle drive device of Japanese Patent Application Publication No. 2008-114848 includes an input member that is drivingly coupled to an internal combustion engine, an output member that is drivingly coupled to wheels, and a differential gear unit having three rotary elements, namely a sun gear, a carrier, and a ring gear. A first rotating electrical machine is drivingly coupled to the sun gear of the differential gear unit, the input member is drivingly coupled to the carrier, and the output member and a second rotating electrical machine are drivingly coupled to the ring gear.
The control device of the vehicle drive device of Japanese Patent Application Publication No. 2008-114848 has a plurality of drive modes including a parallel drive mode (including two shift speeds) and a split mode, which can be switched by switching the axial position of a sleeve member of a dog clutch mechanism. For example, fuel consumption at high vehicle speeds is improved by attaining the drive mode in which the first rotating electrical machine is held stationary and the rotational speed of the input member is increased and transmitted to the output member (the higher speed of the parallel drive mode).
In the vehicle drive device of Japanese Patent Application Publication No. 2008-114848, however, the ring gear and the second rotating electrical machine are always coupled so as to rotate together. Accordingly, the maximum vehicle speed of the vehicle equipped with the vehicle drive device is determined by the maximum rotational speed that can be output from the second rotating electrical machine. In order to allow the vehicle to travel at higher vehicle speeds, it is necessary to further increase the maximum rotational speed of the second rotating electrical machine. However, increasing the rotational speed of the second rotating electrical machine brings new problems such as degradation in torque characteristics, an increase in loss, and the need to increase the withstand voltage of an inverter. It is possible to set the gear ratio to a relatively high fixed value so that the rotational speed of the second rotating electrical machine does not become higher than the maximum rotational speed that can be output from the second rotating electrical machine even at the maximum vehicle speed of the vehicle. However, this reduces torque that is transmitted from the second rotating electrical machine to the output member when the vehicle travels at low vehicle speeds, and a sufficient driving force may not be able to be ensured when the vehicle travels at low vehicle speeds.